This invention relates to the processing of N.T.S.C. colour television signals in digital (e.g. pulse code modulated) form, In particular the invention relates to methods of and apparatus for generating a digital N.T.S.C. colour television signal from an analogue composite N.T.S.C. signal, and regenerating an analogue N.T.S.C. signal from such a digital signal.
It is known that in general where a signal is sampled, a certain minimum sampling frequency is required if all the information conveyed by the signal is to be correctly transmitted. This minimum frequency limit is known as the Nyquist limit, and, for a baseband signal, is equal to twice the highest frequency component in the signal. Any frequency component f.sub.p in the signal will combine with the sampling frequency f.sub.s to generate a component of frequency f.sub.s -f.sub.p. As long as the sampling frequency f.sub.s is at least twice the highest signal frequency f.sub.v, all the components f.sub.s -f.sub.p will be of frequency higher than f.sub.v and can be removed by low-pass filtering. However, if f.sub.s falls below 2 f.sub.v, then some of the components f.sub.s -f.sub.p will fall in the signal frequency band. These unwanted frequency components are called `alias` components.
For transmitting PAL colour television signals, the use of sub-Nyquist sampling frequencies has been described in British patent specification Nos. 1,511,230 and 1,524,749 and 1,580,724. The techniques there described rely on the particular format of the PAL system, and furthermore alias components generated by the PAL chrominance signal must be allowed to pass through a comb filter in the receiver, whereas with N.T.S.C. signals the chrominance alias components should be rejected by this comb filter.
N.T.S.C. signals have proved to be more difficult to digitise with a sub-Nyquist frequency, even in the light of the methods successfully proposed for PAL. For transmitting N.T.S.C. colour television signals, it has been proposed by JOHN P. ROSSI in a paper entitled "Sub-Nyquist-Encoded PCM NTSC Color Television" published in SMPTE Journal Vol.85, No.1, January 1976, pages 1 to 6, to take advantage of the fact that the liuminance and chrominance components tend to cluster around specific frequencies related to the line frequency f.sub.L. That is, the luminance components tend to cluster around integral multiples of the line frequency, i.e. frequencies given by kf.sub.L where k is an integer, and the chrominance components tend to cluster around odd integral multiples of half the line frequency, i.e. frequencies given by (k+1/2) f.sub.L. Rossi proposes the use of a sub-Nyquist sampling frequency of the form (m.+-.1/4)f.sub.L, where m is an integer, together with a comb filter at the receiver. This filter is arranged to remove all frequencies above the highest video frequency f.sub.v, to pass all frequencies below f.sub.s -f.sub.v, where f.sub.s is the sampling frequency, and to act as a comb filter in the range f.sub.s -f.sub.v to f.sub.v. The comb filter response has nulls at frequencies given by (n.+-.1/4)f.sub.L, where n is an integer, thus removing the highest energy alias components, and has minimum attenuation at frequencies given by nf.sub.L /2, which are the central frequencies for the clusters of wanted luminance or chrominance components. Such comb filtering can be achieved by combining video signals from alternate (not adjacent) time sequential television lines, i.e. lines separated by two line scan periods.
With Rossi's sub-Nyquist process, wanted high frequency components occurring at frequencies given by (n.+-.1/4)f.sub.L are removed by the comb filter, but they generate alias components which pass through the comb filter as they occur at frequencies given by nf.sub.L /2. A further improvement in the ratio of wanted-to-alias components is obtained by removing wanted components at (n.+-.1/4)f.sub.L prior to sampling by means of a second comb filter similar to that described above.
As described by Rossi, preferred sampling frequencies for this method of sub-Nyquist sampling are 2 f.sub.sc +1/4 f.sub.L or 2 f.sub.sc -1/4f.sub.L, where f.sub.sc is the chrominance subcarrier frequency; both these frequencies are of the form (m.+-.1/4)f.sub.L since for N.T.S.C. signals, 2f.sub.sc is an odd integral multiple of f.sub.L. Alternatively, the video signal can be sampled at a frequency which remains constant at precisely 2f.sub.sc during any one line period accompanied by a phase shift of 180.degree. at the start of every alternate time sequential line period. This is described by Rossi in a second paper entitled "Sub-Nyquist Sampled PCM NTSC Color TV Signal Derived from Four Times the Color Subcarrier Sampled Signal" read at the International Broadcasting Convention 1978, and published as IEE Conference Publication No.166 at pages 218 to 221. This latter sampling technique, involving a 180.degree. phase shift, has the advantage that the required samples can be extracted at precisely 4f.sub.sc. This sampling frequency has been recommended as a standard.
The main impairments introduced by the N.T.S.C. sub-Nyquist sampling technique described above have proved to be:
(a) reduction of high frequency diagonal luminance resolution, PA1 (b) reduction of vertical chrominance resolution, and PA1 (c) introduction of alias components into the output signal on diagonal high frequency luminance detail or non-vertical chrominance detail.